A conventional technology reads an image by using a sensor array that includes multiple sensor elements lined up in a single row. For example, a magnetic sensor device for reading magnetic information printed on paper with magnetic ink reads the magnetic information by using bridge-connected magnetoresistive elements that have a property that is change of resistance upon application of a magnetic field.
In a magnetic sensor device for detection of a minute magnetic pattern such as that of paper currency, the change of resistance of the magnetoresistive element is minute due to minuteness of the amount of magnetism of the magnetic pattern used for the paper currency. Thus a sensor output signal read from mid-points of the bridge-connected magnetoresistive elements is minute. Thus acquisition of the pattern information requires that the sensor output signal in a low noise state is amplified at a high gain of about 1,000.
An amplifier that achieves high gain and low noise has high current consumption, and due to connection of such an amplifier to each of the magnetoresistive elements, many amplifiers operate, and power consumption increases. Due to such increase, a problem of increased cost occurs with increased capacity of a power supply that supplies power to the device, and heat generated by the device lowers working life of components mounted within the device, posing a problem in that a mechanism is required for dissipation of the heat.
In response to such problems, technology is proposed that suppresses power consumption by setting the amplifiers to an operating state only when reading an image, and by stopping the bias current during a wait state of the circuit when circuit operation is not required (for example, Patent Literature 1). Patent Literature 1 describes an ability to realize outputting and stopping of the bias current by appending a control circuit to a current output terminal of a current mirror circuit.